Loop statements

In Rell, loop statements include the for, while, break, and continue statements, each serving distinct purposes for controlling program flow. This summary provides an overview of these loop statements and their applications.

The for loop

The for loop iterates over a specified range or collection, executing a block of code for each element. For example:

for (x in range(10)) {
    print(x);
}

for (u in user @* {}) {
    print(u.name);
}

In the first for loop:

• x takes on values from 0 to 9 (inclusive) because of range(10), which generates a sequence of numbers from 0 to 9.
• During each iteration, the print(x) statement prints the current value of x.

In the second for loop:

• u iterates over each user entity that matches the condition @* {}, which selects all users without any filtering criteria.
• During each iteration, the print(u.name) statement prints the name attribute of the current user entity.

The expression after in may return a range or a collection (list, set, map).

Can use tuple unpacking in a loop:

val l: list<(integer, text)> = get_list();
for ((n, s) in l) { ... }

The while loop

The while loop repeatedly executes a block of code as long as a specified condition remains true. For example:

while (x < 10) {
    print(x);
    x = x + 1;
}

This loop prints values of x while x is less than 10.

The break statement

The break statement is used to prematurely exit a loop when a certain condition is met. For example:

for (u in user @* {}) {
    if (u.company == 'Facebook') {
        print(u.name);
        break;
    }
}

while (x < 5) {
    if (values[x] == 3) break;
    x = x + 1;
}

In the first for code snippet,

• The for loop iterates over all user entities in the blockchain (specified by user @* {}).
• During each iteration, it checks if the company attribute of the current user entity is equal to 'Facebook'.
• If the condition is met, the print(u.name) statement prints the name attribute of the current user and then the break statement is executed.
• The break statement immediately terminates the loop, meaning that no more iterations will occur even if there are more user entities to process.
• This loop searches for a user whose company is 'Facebook', and as soon as such a user is found and printed, the loop stops.

In the second while code snippet,

• The while loop continues to execute as long as the condition x < 5 is true.
• Within the loop, there's a conditional statement that checks if the value at index x in the values array is equal to 3.
• If the condition is met, the break statement is executed, causing the loop to immediately terminate.
• If the condition is not met, the loop continues to the next iteration, incrementing the value of x by 1.
• This loop iterates through the values array, looking for the value 3. As soon as it finds such a value, the loop stops executing.

In both cases, the break statement is used to exit a loop prematurely based on a specific condition. It provides a way to control the flow of the loop and terminate it early when certain criteria are met.

The continue statement

The continue statement is used to skip the remaining iterations of the current loop iteration and proceed to the next iteration. For example:

for (u in user @* {}) {
    if (u.company == 'BigCompanyCo') {
        continue;
    }
    print(u.name); // Will print every user who does not work at BigCompanyCo.
}

In this example:

• The for loop iterates over all user entities (specified by user @* {}).
• During each iteration, it checks if the company attribute of the current user entity is equal to 'BigCompanyCo'.
• If the condition is met (if the user works at 'BigCompanyCo'), the continue statement is executed.
• The continue statement causes the loop to skip the rest of the code inside the loop block and move on to the next iteration immediately.
• If the condition is not met (if the user does not work at 'BigCompanyCo'), the print(u.name) statement is executed, printing the name attribute of the current user.